Use of natural gas as a fuel source for motor vehicles is on the rise. Because it burns cleaner, natural gas is less harmful to the environment and better for engines than gasoline and other gaseous fuels. Natural gas is readily available in most parts of the world.
The volume of natural gas required to operate a vehicle for a reasonable driving range is too great for the gas to be practically stored on the vehicle in its normal state. As a result, the gas is compressed into a smaller volume and stored on the vehicle in one or more high pressure gas cylinders. When compressed to a volume that exerts a pressure of 3000 psig, natural gas provides about one-fourth the driving range provided by an equivalent volume of gasoline. Although the volume of natural gas can also be reduced by liquification, the resulting liquified natural gas ("LNG") must be cryogenically stored and involves other complications.
Vehicle CNG storage cylinders can be filled with CNG by either a "slow fill" method or a "fast fill" method. In a slow fill method, natural gas is conducted from a utility gas supply (typically at a pressure of about 5-60 psig) to a compressor. After being compressed by the compressor, the gas is conducted directly into the vehicle storage cylinder. Most of the compressors being used at this time deliver the CNG to the vehicle storage cylinder at a pressure between 3000 and 3600 psig and a rate of about 50 scfm. When full, a typical vehicle storage cylinder (e.g., 10 gallon equivalent) maintains approximately 1025 scf of CNG at a pressure of about 3000 psig.
A disadvantage of slow fill methods is the amount of time required for the vehicle gas cylinder to be filled. The rate CNG is provided to the vehicle gas cylinder is dependent upon the size of the compressor, but for a typical 50 scfm compressor it takes approximately twenty minutes to fill a 10 gallon equivalent gas cylinder. In most applications, compressors capable of delivering CNG at a suitable pressure and at a rate faster than about 50 scfm are cost prohibitive or otherwise not practical. As a result, slow fill methods are generally suitable only for refueling fleet vehicles such as school busses and the like that can be filled overnight.
Fast fill methods allow a typical vehicle storage cylinder (e.g., 10 gallon equivalent) to be filled in three to four minutes. This fill rate is achieved by conducting the CNG to the vehicle storage cylinder at a relatively high rate from one or more storage tanks containing a large volume of CNG at a pressure above the pressure required to fill the cylinder, e.g., above 3000 psig. Several vehicle storage cylinders can be filled at the same time. Fast fill CNG systems generally employ a battery of high pressure cylindrical tanks positioned above the ground. A typical fast fill station maintains approximately 30,000 scf of CNG at a pressure between 3200 and 3600 psig. The storage tanks are usually operated in a sequential manner. For example, when the pressure in one tank approaches equilibrium with the pressure in the vehicle storage cylinder(s) being filled, the system switches to a second tank. Once the pressure in the second tank approaches equilibrium with the pressure in the vehicle storage cylinder(s) being filled, the system switches to a third tank and so forth. The compressor operates to refill the storage tanks as they are depleted.
As the demand for CNG as a fuel source for vehicles increases, more and more fast fill CNG service stations will be needed. In order for CNG to be accessible to the general public, such stations will have to be suitable for installation in residential and other urban areas.
By the present invention, a CNG storage and fast fill service system having greater fast fill capability, increased safety features and improved aesthetics is provided.